Conventional heat pipes comprise at least two fluid ducts. The liquid phase of the heat carrier flows from the cool end to the hot end. The evaporated phase of the heat carrier flows from the hot end to the cool end. The first mentioned channel is referred to as the liquid channel. The second channel is referred to as the vapor channel. Means may be provided for transporting bubbles that may be present in the liquid channel into the vapor channel.
Such heat pipes for the transport of heat are particularly useful in space technology. The heat carrier is normally ammonia which is evaporated at the heat absorbing end of the pipe and the vapor is transported to the heat discharging end of the pipe which is the condenser end, whereby the heat given off by the vapor as it is being condensed is discharged to the environment. The condensate or liquid flows back again to the evaporator end of the pipe by capillary action. The vapor flow from the evaporator end to the condenser end is maintained by a pressure difference between these two ends whereby the vapor flow is a pressure flow. Different radii of curvature along the boundary surface or wall between the liquid channel and the vapor channel at the evaporator end on the one hand, and at the condenser end on the other hand, and the capillary forces caused thereby impose a pressure difference in the direction toward the evaporator end and this pressure difference maintains the flow. The resulting flow velocity depends on the equilibrium that is established between the pressure loss due to frictional forces and the effective capillary forces.
Modern high performance heat pipes are capable of transporting substantial heat quantities over substantial distances even at relatively small temperature differences between the hot, evaporating end, and the cold or condensing end of the heat pipe. For example, one kilowatt can be easily transported over distances from 1 to about 20 m. Higher heat quantities have been transported over shorter distances.
Comparing conventional high performance heat pipes with other conventional heat pipes, the higher performance of the former is achieved in that the transport of the liquid takes place through channels of differing dimensions. In the vaporization zone a multitude of very small channels having geometries for capillary action are used in order to achieve substantial driving capillary forces. In the condensating zone and in the section between the evaporating and condensing zones, namely in the transport zone, the transport takes place through few flow channels and if suitable even in a single channel with a relatively large diameter. Such a large diameter channel may also be referred to as an artery. The just described structure minimizes pressure losses due to frictional forces. As a result, a substantially increased fluid mass flow is achieved even though the capillary forces remain the same. Simultaneously, a substantially increased heat transfer or heat flow is achieved due to the improved mass flow.
In operating such high performance heat pipes, however, a substantial problem is encountered. Such a problem is caused by vapor bubbles of the heat carrier fluid or by gaseous noncondensible foreign matter. Bubbles and noncondensible matter impair the function of a heat pipe substantially or may even interrupt the operation. Such bubbles or foreign matter may have been present inside the heat pipe already at the time of starting the operation and their presence may have been completely accidental. Such impairments may also be caused by an operational overloading of the heat pipe, for example, by superheating the evaporation end of the pipe causing a short duration, temporary drying of the evaporation zone. Resulting bubbles can interrupt the transport of the heat carrier fluid to the hot end of the pipe so that the hot end even dries further, thereby blocking the further function of the heat pipe.
Two conventional heat pipes are described in "Heat Pipe Design Handbook", Volume 1, by B+K Engineering Incorporated, Towson, Md., 21204 (U.S.A.), pages 149 and 152. These conventional heat pipes include devices for the removal of bubbles and thus avoiding the blockage of the desired flow by the gas bubbles. In one instance, gas bubbles are avoided by venting bores in the separation wall between the artery and the vapor channel. In the other instance, the gas bubbles are avoided by a suction nozzle arranged in the transport area for the vapor. The suction nozzle functions simultaneously as a jet pump for sucking off gas bubbles in the artery through a suction pipe.
The arrangement of venting holes in the wall of the artery has the disadvantage that during the operation of the heat pipe the pressure in the vapor channel is substantially higher than in the artery so that for transferring gas bubbles out of the artery into the vapor channel, the operation of the heat pipe must be interrupted. However, during such interruption the venting bores are blocked by liquid bridges which must first evaporate before the gas bubbles can pass through the venting bores. As a result, such interruptions of the operation of the heat pipe require relatively long time periods before the heat pipe can become operational again.
With regard to the second conventional devices for the removal of bubbles by a suction nozzle or venturi nozzle, there is the disadvantage that, in case there is no gas bubble within the suction range of the suction nozzle, a small quantity of heat carrier fluid is collected from the artery into the suction pipe. If now a gas bubble does appear in front of the suction inlet, it is necessary to first suck in the liquid quantity out of the suction pipe to be able to also remove the gas bubble. The result is a substantial pressure loss in the flow in the suction pipe. As a result, the pressure reduction caused thereby in the suction nozzle is correspondingly substantial. Thus, the nozzle must have a relatively large reduction in the cross-sectional flow area. Such a reduction in turn leads to a substantial impairment of the vapor flow, due to the pressure loss and thus to a substantially reduced effectiveness of the heat pipe.